This invention relates in general to analytical instruments and, in particular, to instruments employing a small cavity to reduce noise and other artifacts caused by resonances of structural elements of the instruments.
With the advent of instruments such as the scanning tunneling microscope (STM), it is now possible to investigate the structure, spectra and dynamics of biological molecules and membranes as well as other substances at the atomic or molecular level. While more than a thousand STM's have been in operation and the instrument has sparked great interest in spectroscopy, the actual headway that has been made in this area remains rather modest. Thus, Bob Wilson and co-workers at IBM Almaden have made some progress in distinguishing closely related adsorbed surface species in STM images. G. Meijer et al., Nature 348, 621 (1990). In "Non-Linear Alternating-Current Tunneling Microscopy," Kochanski, Physical Review Letters, 62(19):2285-2288, May 1989, a method for scanning tunneling microscopy is described, where a non-linear alternating current (AC) technique is used that allows stable control of a microscope tip in close proximity to insulating surfaces where direct current (DC) tunneling is not possible.
The STM has a counter electrode on which the sample to be investigated is placed and another electrode in the shape of a microscope probe with a tip placed a small distance from the sample surface. A DC or a low frequency AC signal is then applied across the pair of electrodes. The probe tip is then moved across the sample surface in a scanning operation and the changes in the current or voltage across the electrodes are monitored to detect the characteristics of the sample. Where the probe tip of the STM applies an alternating current (AC) signal to the sample, the STM is referred to as an ACSTM.
A number of specific implementations of the scanning tunneling microscope have been proposed. See, for example, "A Versatile Microwave-Frequency-Compatible Scanning Tunneling Microscope," by Stranick and Weiss, Rev. Sci. Instrum., 64(5):1232-1234, May 1993; "Coarse Tip Distance Adjustment and Positioner for a Scanning Tunneling Microscope," by Frohn et al., Rev. Sci, Instrum., 60(6):1200-1201, June 1989; a product brochure from Besocke Delta Phi GmbH of Juelich, Germany, entitled "The Beetle STM - A Versatile, UHV Compatible Scanning Tunneling Microscope," and "An Easily Operable Scanning Tunneling Microscope," by Besocke, Surface Science, 181:145-153, 1987.
In the existing designs of the ACSTM, artifacts are often present in the output signal of the ACSTM, where the artifacts are caused by resonance effects of the structure of the microscope and its surroundings. This is particularly the case where the AC signal provided by the ACSTM is at high frequencies such as at microwave frequencies. The use of microwave frequencies in the ACSTM is taught, for example, in U.S. Pat. No. 5,268,573. It is therefore desirable to provide an improved ACSTM where such resonance effects are reduced to improve the signal-to-noise ratio.